As one of a communication system for the third generation wireless communications (standardized by ITU-T as International Mobile Telecommunications-2000, or IMT-2000), the W-CDMA (Wideband Code. Division Multiple Access) system has been adopted. In this W-CDMA, various techniques are introduced to improve communication quality (wireless transmission quality), which include soft handover (site diversity), site selection diversity transmit power control (SSDT), and closed-loop transmission power control.
The soft handover is a technique in which a mobile station is simultaneously connected with a plurality of base stations through wireless links, and receives signals from the plurality of base stations using the RAKE receivers. At the time of the soft handover, the entire plurality of base stations performing the soft handover transmit signals on Dedicated Physical Channels (DPCH), and the mobile station receives the DPCH signals from the plurality of base stations.
The SSDT is a method for power control performed at the time of the soft handover, to solve a problem of increased interference on a downlink (i.e. a link directed to a mobile station from a base station) produced by the identical DPCH signals being transmitted to the mobile station from the plurality of base stations at the time of the soft handover. The mobile station selects one of the base stations performing the soft handover as a primary cell, whereas other base stations are determined as non-primary cells. Only the primary cell transmits signals on a Dedicated Physical Data Channel (DPDCH) in the DPCH, and non-primary cells do not transmit any DPDCH signals. Among the DPCH signals, Dedicated Physical Control Channel (DPCCH) signals are transmitted for the entire base stations performing the soft handover.
In the SSDT method, a mobile station measures the received signal code power (RSCP) of a Common Pilot Channel (CPICH) transmitted with constant power from each base station. The mobile station selects base stations of which measurement result is higher than the predetermined threshold as soft handover candidate. Among these soft handover candidates, the mobile station selects the base station producing the maximum RSCP as primary cell. By changing (updating) the primary cell at high speed, the mobile station can receive the DPDCH with better reception quality.
The closed-loop transmission power control is a power control method performed in the following way: Both a mobile station and a base station measure reception signal quality (i.e. transmission signal quality in terms of the transmission side). Depending on the measurement result, a transmit power control (TPC) command is transmitted to a transmission side so that a reception side can receive a signal with desired quality. The transmission side then controls the own transmission power based on the TPC command. This control method aims to solve the far-to-near problem and reduce an effect caused by fading fluctuation. As a measurement criterion for the receive signal quality, the SIR (signal-to-interference power ratio) is applied.
Here, in the conventional SSDT, particularly in an inner-loop control of the transmission power control on the downlink, a mobile station measures the SIR of the DPCCH received from a primary cell, compares the measurement value with a target SIR value, and generates a TPC command in accordance with the comparison result. The mobile station then transmits this TPC command to both the primary cell and the non-primary cells through the uplinks (links directed to the base stations from the mobile station). According to the TPC command, the primary cell controls transmission power of the DPDCH and the DPCCH on the downlink. Meanwhile, the non-primary cells control the transmission power of the DPCCH on the downlink, but do not control the transmission power of the DPDCH.
FIG. 6 shows a state of transmission power control on the downlink at the time of the SSDT. A base station selector 102 provided in a mobile station 100 measures the RSCP of the CPICH transmitted from n base stations 2001-200n (where n is integer no less than 2), and selects the base station producing the maximum RSCP value as primary cell. The selection result of the primary cell is transmitted to base stations 2001-200n on uplink feedback information (FBI), in which identification information of the base station indicating the primary cell is included. Thus, each base station 2001-200n can identify whether the base station of interest is the primary cell or the non-primary cell.
An SIR measurement section 103 provided in mobile station 100 measures the SIR of the DPCCH transmitted from the base station which has been selected as primary cell by base station selector 102, among the DPCCH transmitted from base stations 2001-200n. SIR measurement section 103 then feeds the measurement result to a TPC bit generator 104. TPC bit generator 104 compares the measured SIR with a target SIR having been set in advance, and generates a TPC command based on the comparison result. The generated TPC command is transmitted to base stations 2001-200n.
In base stations 2001-200n, an FBI bit extractor 204 extracts, from the received data, identification information of the base station having been selected as primary cell. Based on this base station identification information, a switcher (SW) 203 determines whether or not the base station of interest is selected as primary cell. If the base station of interest has been selected as primary cell, the base station concerned outputs a DPDCH data to a power controller 202, whereas if the base station concerned is not selected as primary cell, the base station concerned does not output any DPDCH data to power controller 202.
Meanwhile, a TPC bit extractor 205 extracts the TPC command from the received data, and then feeds the TPC command to power controller 202. Power controller 202 controls the transmission power of the DPCCH according to the TPC command. Power controller 202 further controls the transmission power of the DPDCH in case the DPDCH data is supplied from switcher (SW) 203 according to the TPC command. These power-controlled channel data are transmitted to mobile station 100.
The power control by power controller 202 is performed using the same control method, irrespectively of whether the base station concerned is the primary cell or the non-primary cell, according to the same TPC command (that is, the same increase/decrease amount of power based on the same increment/decrement). FIG. 7 shows such a conventional power control method in a tabular form. Both the primary cell and the non-primary cell increase the transmission power by 1 dB when the TPC command indicates ‘1’, or decrease the transmission power by 1 dB when the TPC command indicates ‘0’.
As such, in the conventional SSDT method, the primary cell selection is performed independently of the transmission power control. Namely, the primary cell selection is determined being referenced from the RSCP of the CPICH, whereas the transmission power control is performed by use of the TPC command determined by referencing the SIR of the DPCCH. Moreover, the TPC command is transmitted on each time slot bases, and therefore the transmission power control is updated at each time interval T of the time slot (for example, T=0.667 ms). In contrast, the primary cell selection information is transmitted using no less than three time slots, and therefore the primary cell is updated at time intervals three times as long as the time interval T.
Now, according to the aforementioned method in which the primary cell selection and the transmission power control are performed independently, there lies a problem that an optimal primary cell selection cannot always be guaranteed.
More specifically, although the base station transmitting the DPDCH signals with better quality has to be selected, according to the conventional method, the criteria applied for the transmission power control which effects the communication quality is different from the criteria for the primary cell selection. As a result, there may be cases that a base station which provides better communication quality be not selected as primary cell.
Also, because the period of updating the primary cell is longer than the period of updating the transmission power control, there may be cases that updating the primary cell cannot follow the change of transmission power. As a result, a base station providing larger transmission power may not be selected as primary cell.
Further, because the primary cell update period is long, there may also be cases that updating the primary cell cannot follow fading fluctuations. This may also impede to receive the DPDCH signal transmitted from a base station providing larger transmission power.
Also, conventionally, as having been illustrated in FIG. 7, both the primary cell and the non-primary cells perform identical transmission power control based on the common TPC command, which may possibly make it difficult to switch over from the primary cell to a non-primary cell. This may reduce effect of high-speed cell selection obtained by a rapid switchover of primary cell to more optimal base station.
Moreover, because generally lower communication quality is provided by a non-primary cell than by the primary cell, the transmission error rate for a TPC command may possibly be increased on the uplink also. In such a case, there arises a problem that the base station may perform transmission power control based on an incorrect TPC command. As a result, greater transmission power difference than transmission loss difference may be produced between a plurality of base stations which are soft handover candidates. This may produce increased interference among the downlinks.
To cope with this problem, a method has been proposed in the Technical Report of IEICE, RCS 2000-164, published by the IEICE (the Institute of Electronics, Information and Communication Engineers). According to the proposed method, which has been referred to as SIDTPC (site independent diversity transmit power control), a mobile station measures the SIR of the signal from each base station after the RAKE receiver, and generates the TPC command so that each base station can perform independent transmission power control.
However, according to this method having been proposed, it becomes necessary to provide a large amount of bits in a TPC command to be transmitted in the uplink DPCCH, which becomes as many as the number of base stations, as compared with the conventional method. Or, in order to make the number of the TPC command bits identical to the conventional method, the transmission power control period becomes longer, which may deteriorate capability to follow fading fluctuations.